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Hydraulically discrete fascicles in skeletal muscle

The following is purely speculative and relates to skeletal muscle and medial arch supports .

It has been shown experimentally that transverse compression of skeletal muscle can ,under some circumstances ,cause the lateral force generated by the muscle during contraction to be reduced (1) .
It seems reasonable to think that if the more superficial parts of a muscle such as the pectoralis major contract to apply a force then the deeper fibres of the muscle will be subject to transversal loading .
For me this raises the question of whether the transversal loading of the deeper fibres by the more superficial reduces the contribution that the deeper fibres make to the overall force production of the muscle or if the anatomy of skeletal muscle allows the deeper fibres to function fully without a reduction in lateral force production .

Skeletal muscle consists of fibres covered by a thin membrane called the endomysium . Fibres are further arranged into bundles or fascicles which are covered by a relatively thick membrane called the perimysium which is generally regarded as a structure which shapes and organizes the muscle fibres and also allows the transmission of lateral contractile movements .
If the perimysium is sufficiently impermeable then might it be possible that each fascicle is able to function as a hydraulically discrete unit with little or no reduction in force production regardless of the transversal forces generated by more superficial parts of the muscle .

The following simple analogy may help to explain the idea -

-Imagine a fish tank 20 cm long, 4cm wide and 30 cm high.
Next imagine you place 10 small eels of about 20cm in length in the tank and add some water so that it just covers the eels .Each eel represents a single muscle fibre and these are free to move around although for the sake of the analogy lets assume the eels maintain a length wise orientation along the tank .

Now imagine adding a 10kg weight lowered on top of the eels . The weight would be a good fit for the tank with just enough room at the sides for water to flow easily between the tank and the weight .

The weight will bear down on the eels ,squeezing away the water around them and pinning the eels flat . This represents the effects of transverse compression on the deeper fibres of a large skeletal muscle with curved fibres (2) during contraction of that muscle .

So does the anatomy of skeletal muscle provide any means by which the deeper fibres of a muscle can function normally under transverse load ? Possibly .

Firstly take our eel, water, tank system but this time place the eels and the water in a latex bag ,seal the bag and place it in the tank .

Next lower the weight down on top of the bag full of eels and this time the eels are not crushed but instead are free to move around in the pressurized water . The latex bag represents the perimysium .
The idea is that so long as a skeletal muscle fascicle is bounded on all sides by other fascicles,under similar pressure a hydraulically discrete unit capable of elastic deformation can exist which would allow the muscle fibres within the fascicle to contribute fully to the contraction of the muscle .

( Obviously the above analogy is a gross simplification of a far more complex situation.)

A recent study (3) has shown that work partitioning of transversally loaded muscle can reduce the lateral force generated by the muscle and greatly reduce the rate of force production . However I feel that the experimental rig used by Siebert and his co workers does not accurately represent the type of transverse loading applied by the contacting superficial fibres of a muscle to the deeper layers of the muscle .

Going back to the eels/water /tank/bag analogy lets now assume that the weight used to represent the pressure from superficial fibre contraction now has a distinct non compressible ridge running transversely across its base at its midline .

Now when the weight is lowered onto our bag of eels (fascicles and muscle fibres ) the ridged area will press into the bag in its mid section bypassing the protective hydraulic system and applying direct pressure from above onto the eels reducing their ability to move . I believe this is what may be happening in the experiments conducted by Siebert . The” hydraulically discrete fascicle system “ proposed can only function if the fascicles are completely surrounded by fascicles at similar pressures .

Change in muscle shape during concentric contraction

When a muscle such as the pectoralis contracts to apply force it shortens and the muscle belly cross section increases . So how can this increase in muscle belly volume happen with as little work partitioning as possible . Again I feel the answer may lie in part in the “Hydraulically discrete fascicle system “.

Back to balloons !

If a long modelling type party balloon is filled with water and sealed then the pressure of the water will be the same at all points inside the balloon . However ,examination of the balloon will reveal the ends are under far less tension than than the midsection since the ends have a reduced radius . This is explained by Laplace’s law .

The ends of balloon can be pushed in fairly easily resulting in a slight increase in the area of the midsection .Perhaps then when a fascicle contracts the greatest movement is at the ends of the fascicle allowing force to be transferred to the tendon whilst reducing work partitioning and allowing a degree of muscle gearing (Pascal ) in the midsection .

So what does all this have to do with footwear .Well as Sweeney et al (4) confirm orthoses with a medial arch support cause the plantar muscular to be compressed and so I believe the functioning of medial arch supports and the volume and contractile state of the plantar intrinsics are highly inter related .